Terrestrial Impact Craters

Compiled by Christian Koeberl and Virgil L. Sharpton

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Introduction

Impact craters are geologic structures formed when a large meteoroid (asteroid or comet) smashes into a planet or a satellite. All the inner bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The surfaces of the Moon, Mars, and Mercury, where other geologic processes stopped millions of years ago, record this bombardment clearly. On the Earth, however, which has been even more heavily impacted than the Moon, craters are continually erased by erosion and redeposition as well as by volcanic resurfacing and tectonic activity. Thus only about 120 terrestrial impact craters have been recognized, the majority in geologically stable cratons of North America, Europe, and Australia where most exploration has taken place. Spacecraft orbital imagery has helped to identify structures in more remote locations for further investigation.

Meteor Crater (also known as Barringer Crater), Arizona, was the first-recognized terrestrial impact crater (slide #6). It was identified in the 1920s by workers who discovered fragments of the meteorite impactor within the crater itself: Several other relatively small craters were also found to contain impactor fragments, and, for many years, these remnants were the only accepted evidence for impact origin. However, scientists have come to realize that pieces of the impactor often do not survive the collision intact.

In massive events caused by a large impactor, tremendous pressures and temperatures are generated that may vaporize the meteorite altogether or may completely melt and mix it with melted target rocks . Over several thousand years, any detectable meteoritic component may erode away. In some cases nonterrestrial relative abundances of siderophile elements can be detected in the impact melt rocks within large craters—a chemical signature of the meteorite impactor.

Since the 1960s, numerous studies have uncovered another physical marker of impact structures, shock metamorphism. Certain shock metamorphic effects have been shown to be uniquely and unambiguously associated with meteorite impact craters; no other earthly mechanism, including volcanism, produces the extremely high pressures that cause them. They include shatter cones , multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass , and high-pressure mineral phases such as stishovite . All known terrestrial impact structures exhibit some or all of these shock effects.

Impact craters are divided into two groups based on morphology: simple craters and complex craters. Simple craters are relatively small with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape (slide #1, top; Fig. 1a). In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20) (slide #1, bottom; Fig 1b). The diameter at which craters become complex forms depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2 to 4 km (depending on target rock properties); on the Moon, at 1/6 Earth’s gravity, the transition diameter is 15 to 20 km.

Fig. 1. Schematic cross sections of (a) simple and (b) complex impact craters showing idealized near-surface distribution of impact features. Simple crater forms are typical of structures with rim diameters of less than about 4 km; central uplifts, shallow floor depths, and slumped rims characterize complex craters with diameters larger than about 4 km. The surface expression of the central uplift is typically a single peak in craters with diameters between approximately 4 and 50 km. Larger impact structures can have complex ring-shaped uplifts.

The central peak or peak ring of the complex crater is formed as the initial (transient) deep crater floor rebounds from the compressional shock of impact. Slumping of the rim further modifies and enlarges the final crater. Complex structures in crystalline rock targets will also contain coherent sheets of impact melt atop the shocked and fragmented rocks of the crater floor. On the geologically inactive lunar surface, this complex crater form will be preserved until subsequent impact events alter it. On Earth, weathering and erosion of the target rocks quickly alter the surface expression of the structure; despite the crater’s initial morphology, crater rims and ejecta blankets are quickly eroded and concentric ring structures can be produced or enhanced as weaker rocks of the crater floor are removed. More resistant rocks of the melt sheet may be left as plateaus overlooking the surrounding structure.

Large terrestrial impacts are of greater importance for the geologic history of our planet than the number and size of preserved structures might suggest. For example, recent studies of the Cretaceous-Tertiary boundary , which marks the abrupt demise of a large number of biological species including dinosaurs, revealed unusual enrichments of siderophile elements and shock metamorphic features that are markers of meteorite impact events. Most researchers now believe that a large asteroid or comet hit the Earth at the end of the Cretaceous Period 66 million years ago. An environmental crisis triggered by the gigantic collision contributed to the extinctions. Based on apparent correspondences between periodic variations in the marine extinction record and in the impact record, some scientists suggest that large meteorite impacts might be the metronome that sets the cadence of biological evolution on Earth—an unproven but intriguing hypothesis.

Fig 2. Location of structures shown in this slide set. Number corresponds to slide number.

This slide set presents orbital and aerial photographic views of 16 proven or suspected terrestrial impact structures that have distinct surface expression. Several examples of impact structures on Earth’s planetary neighbors are included to show the fundamental role impact plays in shaping planetary surfaces. These relatively well-preserved extraterrestrial craters provide an important reference to understanding the more eroded impact features on Earth. The terrestrial structures we have chosen represent a compromise between those with the best surface expression and those that represent a diversity of age, size, and appearance as the craters are reworked by geologic processes. The freshest craters are presented first, followed by those that exhibit more ambiguity and complexity. We have included two examples of features that have the appearance of impact structures but have not yet been shown to contain any of the diagnostic chemical or physical markers of impact events. Finally, as a cautionary example of the limitations of using images to identify impact craters on Earth, we include two structures that are clearly not of impact origin but nonetheless have some of the surface characteristics of impact features.

The locations of all structures are shown in Fig. 2 (slide #26); the numbers are indexed to the slide caption number.

Slide Captions

1. Crater Cross Sections

Schematic cross sections of (a) simple craters and (b) complex craters showing idealized form, structure, and distribution of impact units. Simple crater forms are typical of structures with rim diameters of less than about 4 km (on Earth); above this transition diameter craters are characterized by complex morphologies exhibiting central uplifts, shallow floor depths, and slumped rims. For complex craters with diameters of about 4 to 50 km the central uplift occurs as a single peak. Larger impact structures can have complex ring-shaped central uplifts.

2. Isidorus D, Lunar Highlands

Diameter: 15 km

This oblique view looking north, taken with the panoramic camera on Apollo 16, shows a typical, simple bowl-shaped crater on the Moon. Evidence of avalanching and slope collapse are clearly visible on the inner walls of the crater. Streaks on the left wall appear to be avalanche scars; along the southeast part of the crater wall, many short irregular benches or narrow terraces mark the tops of slump blocks. The sharp break in slope, marking the rim crest, is clearly visible along the southeast portion of the structure. On Earth, the transition from simple craters to complex craters occurs at smaller diameters than on the Moon because of Earth’s higher gravity. (Apollo lunar photograph AS16-4502(P).)

3. Theophilus, Lunar Central Highlands

Diameter: ~100 km

Theophilus is a relatively young crater situated on the Kant plateau, an elevated area in the central highlands near Mare Nectaris. In this oblique view looking south, part of Nectaris is visible as the smooth, dark area near the horizon at the left edge. Theophilus has the ruggedly terraced walls and central peak protruding through a relatively shallow floor characteristic of fresh complex impact structures. Beyond the sharp structural rim are the relatively bright hummocky deposits of the ejecta blanket showing a subtle radial scour texture particularly evident in the lower right quadrant of the image. The scouring is produced as blocks ejected from the crater plough into the surface of the growing ejecta blanket and surrounding target rocks and testifies to the erosional capabilities of meteorite impact. Just to the right of the structure is an older impact structure that has been partially obliterated by the impact event that produced Theophilus. (Apollo lunar photograph AS16-0692(M).)

4. Yuty, Northern Hemisphere of Mars

22°12'N, 34°W; diameter: 19 km

In addition to its dominant central peak and pronounced wall terracing, this young complex crater displays a striking multilobate ejecta blanket that is common on Mars. (On Mars, craters greater than approximately 10 km exhibit complex forms.) The favored model for the lobate ejecta pattern is through fluidized flow: Excavation and heating of ice in the target material causes a ground-hugging surge deposit, rather than aerial ejection along ballistic trajectories. The visibility of the preexisting crater just outside Yuty’s rim indicates that these ejecta deposits are relatively thin. (Viking Orbiter image 003A07.)

5. Complex Craters, Venus

This radar image, obtained by the Magellan spacecraft on September 17,1990, shows a small portion of Lavinia Planitia centered on 27°S, 339°E. The image has been colored to indicate variations in surface elevation. There is almost 1 km of topographic relief in this region; red signifies the highest surfaces and dark blue the lowest. In such radar images, bright areas are usually rough and blocky and dark regions are smooth. Three large meteorite impact craters, with diameters of 35 to 65 km surrounded by bright (rough) ejecta blankets, dominate the landscape. All the craters appear to be partially filled with dark (smooth) units, which are probably lavas that have risen to the surface through the extensive network of fractures produced by the impact. Fluidized material appears to have flowed northward from the largest crater down a shallow grade for more than 200 km, although it is unclear at present whether this flow was produced by the impact event or by later volcanic activity. These venusian structures do not seem to exhibit the broad terrace zones typical of complex structures on smaller planets, although they could be obscured by the lavas inside the craters. Venus is particularly important for understanding terrestrial cratering because Earth and Venus have similar gravitational constants and both have substantial atmospheres. (Magellan image MGN P-36711 )

6. Meteor Crater, Arizona

35°02'N, 111°0 1'W; rim diameter: 1.2 km.
Age: 25,000 years

The origin of this classic simple meteorite impact crater was long the subject of controversy. The discovery of fragments of the Canyon Diablo meteorite, including fragments within the breccia deposits that partially fill the structure, and a range of shock metamorphic features in the target sandstone proved its impact origin. Target rocks include Paleozoic carbonates and sandstones; these rocks have been overturned just outside the rim during ejection. The hummocky deposits just beyond the rim are remnants of the ejecta blanket. This aerial view shows the dramatic expression of the crater in the arid landscape. (Aerial photo courtesy of D. Roddy.)

7. Wolf Creek, Western Australia

19°10'S, 127°47'E; rim diameter: 0.85 km
Age: 300,000 years

Wolf Creek is a relatively well-preserved crater that is partly buried under windblown sand. The crater is situated in the flat desert plains of north-central Australia. Its crater rim rises ~25 m above the surrounding plains and the crater floor is ~50 m below the rim. Oxidized remnants of iron meteoritic material as well ,as some impact glass have been found at Wolf Creek. This photograph is a south-looking, oblique aerial view of the crater. (Aerial photo courtesy of V L Sharpton.)

8. New Quebec, Quebec, Canada

61°17'N, 73°40'W; rim diameter: 3.4 km
Age: 1.4 million years

This aerial view looks east over the 250-meter-deep circular lake that fills the New Quebec Crater, a relatively large, well-preserved simple crater. The rocks involved in this impact event are ancient and strongly deformed gneisses of the Precambrian shield. The jumbled and outwardly tilted rocks comprising the rim rise as much as 160 meters above the surrounding countryside. A meteoritic origin was first proposed in 1949 based on its morphological similarity to Meteor Crater in Arizona (slide #6). This was confirmed much later when diagnostic evidence of shock metamorphism was discovered in the minerals from gneiss samples collected from within the crater. While the ejecta blanket has been removed by erosion, some isolated melt rocks have been found up to 2 kilometers from the crater rim. (Photograph courtesy of Blyth Robertson)

9. Roter Kamm, South West Africa/Namibia

27°46'S, 16°18'E; rim diameter: 2.5 km
Age: ~3.5 million years

Located in the Namib Desert, the raised crater rim is clearly visible against darker background vegetation. Target rocks include primarily Precambrian crystalline rocks and modest amounts of younger sedimentary rocks. Outcrops of impact melt breccias are found exclusively on the crater rim. The crater floor is covered by broad, shifting sand dunes. This slide shows an oblique view of the crater, from about 150 m above ground looking southeast. (Aerial photo courtesy of W. U. Reimold.)

10. Roter Kamm

In this shuttle view, the narrow, extremely circular rim of the crater is a prominent landmark in the southern Namib Desert. (Space shuttle photograph 61C-40-001.)

This shuttle image shows a winter view of the Mistastin Crater, a heavily eroded complex structure. Eastward-moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central 10 km of the structure. Horseshoe Island, in the center of the lake, is part of the central uplift and contains shocked Precambrian crystalline target rocks. Just beyond the margins of the lake are vestiges of the impact melt sheet that contains evidence of meteoritic contamination. Other evidence of impact includes shatter cones, planar features in quartz and feldspar, and diaplectic glasses. (Space shuttle photograph 61A-34-093.)

The Manicouagan impact structure is one of the largest impact craters still preserved on the surface of the Earth. This shuttle oblique view looking south shows the prominent 70-km-diameter, ice-covered annular lake that fills a ring where impact-brecciated rock has been eroded by glaciation. The lake surrounds the more erosion-resistant melt sheet created by impact into metamorphic and igneous rock types. Shock metamorphic effects are abundant in the target rocks of the crater floor. Although the original rim has been removed, the distribution of shock metamorphic effects and morphological comparisons with other impact structures indicates an original rim diameter of approximately 100 km. (Space shuttle photograph 51B-43-060.)

13. Manicouagan

The moderately eroded, central part of the structure (the plateau surrounded by the lake) is partly covered by impact melts and contains shattered rocks and several uplifted peaks about 5 km north of the center. The vast quantity of data obtained on the melt sheet and the underlying target rocks make Manicouagan the most intensively studied large complex impact structure in the world, and it is the primary source of ground-truth data for understanding the cratering process and the substructural configuration of large complex craters on other planets. (Space shuttle photograph STS39-73-044.)

Twin impact craters, which are formed simultaneously by two separate but probably related meteorite impacts, are very rarely recognized on Earth. This pair is situated in crystalline bedrocks of the Canadian shield . The larger Clearwater Lake West (left) shows a prominent ring of islands that has a diameter of about 10 km. They constitute a central uplifted area and are covered with impact melts. The central peak of the smaller Clearwater Lake East (right) is submerged. (Space shuttle photograph 61A-35-86.)

This crater consists of a near-circular bay, about 5 km wide and 220 m deep, in the otherwise shallow Reindeer Lake. Such deep circular lakes are unusual in this region, which is dominated by the shallow gouging of glacial erosion. The circular shoreline, at a diameter of 11 km, is partially surrounded by a ridge with heights to 100 m above the lake surface. The diameter of this ridge, ~13 km, is likely the outer rim of the impact structure. The structure was formed in Precambrian metamorphic crystalline rocks with a conspicuous northwest trending fabric. Although not obvious from the surface, Deep Bay is a complex impact structure with a low, totally submerged central uplift. Samples obtained in the 1960s from drilling into the central structure revealed shocked and fractured metamorphic rocks flanked by deposits of allochthonous , mixed breccias. (Space shuttle photograph 41G-33-36.)

This crater is situated in crystalline bedrocks of the West African Shield and is filled almost entirely by Lake Bosumtwi. Chemical, isotopic, and age studies demonstrate that the crater is the most probable source for the Ivory Coast tektites , which are found on land in the Ivory Coast region of central Africa and as microtektites in nearby ocean sediments. In this photo the crater lake is partly obscured by clouds. (Space shuttle photograph 51I-39-031.)

This highly eroded structure is situated just south of the MacDonnell Ranges (upper part of the picture) in the arid Missionary Plain in the Northern Territories, Australia. Although it could be mistaken for the crater rim, the central ring of hills (5 km diameter) results from differential erosion of the central uplift within this large complex crater. The rim itself has been eroded and is no longer visible, but the circular, grayish colored drainage system outside the inner ring of hills probably marks the original extent of the structure before erosion. (Space shuttle photograph 41D-41-028.)

The Vredefort structure is located near the center of the Witwatersrand Basin, about 100 km from Johannesburg. It is expressed as a central core about 40 km in diameter that is composed of old crystalline rocks. The core is surrounded by a deformed collar of uplifted and overturned younger sediments and lavas. Much of the structure is buried by younger flat-lying sediments resulting in the arcuate shape observed in this image. The diameter of the collar rocks is approximately 80 km, but reconstructions based on the distribution of shock metamorphic effects suggest an original crater diameter of 140 km. Formed almost 2 billion years ago, it is one of the oldest recognized impact structures on Earth. (Space shuttle photograph S08-35-1294.)

The deeply eroded Spider structure occurs within sedimentary rocks of the semi-arid Kimberley plateau, northwestern Australia. The weblike radiating pattern of ridges that inspired the structure’s name is approximately 5 km wide and is most probably the central uplift of a large complex impact crater. (Space shuttle photograph S08-42- 2191.)

This crater, in the desert of Western Australia, consists of a granitic uplifted core, about 10 km diameter, surrounded by a dark crescent-shaped inner ring unit. An outer ring of Precambrian sediments has a diameter of about 20 km. The appearance of this impact structure is complicated by salt deposits (light units) produced by shallow lakes that seasonally fill the depressions and evaporate. (Space shuttle photograph 41D-42-039.)

21. Kara-Kul, Tadzhikistan

38°57'N, 73°24'E; rim diameter: 45 km
Age: &lt 10 million years

The spectacular Kara-Kul structure is readily apparent in this oblique view. Partly filled by the 25-km-diameter Kara-Kul Lake, it is located at almost 6000 m above sea level in the Pamir Mountain Range near the Afghan border. Only recently have impact shock features been found in local breccias and cataclastic rocks. (Space shuttle photograph 51 F-35-080.)

22. Ouarkziz, Algeria

29°00'N, 07°33'W; rim diameter: 4 km
Age: &lt 70 million years

This structure is situated in sedimentary rocks in the rocky desert of northwest Algeria. It displays a well-defined ring that is partly open to the south. The impact origin of the crater has not yet been established. (Space shuttle photograph 41C-31-1032.)

23. Ramgarh, India

25°20'N, 76°37'E; rim diameter: 5.5 km
Age: unknown

This structure is situated in a semi-arid region in eastern Rajasthan. A ring of hills of about 3 km diameter and a small central peak are conspicuous. Although evidence of shock metamorphism that would prove an impact origin has not yet been presented, the surface morphology suggests that this is a likely impact crater. (Space shuttle photograph STS20-44-005.)

24. Gross Brukkaros, South West Africa/Namibia

25°52'S, 17°45'E; rim diameter: 3 km
Age: 82 million years

This structure is not an impact crater, but was created by a subsurface volcanic explosion. The circular calderalike structure is made of sediments that were cemented by hydrothermal activity. Its appearance illustrates that sometimes nonimpact features may look similar to impact structures. (Space shuttle photograph STS41-151-270.)

25. Richat Structure, Ouadane, Mauritania

21°04'N, 11°22'W; outer ring diameter: 38 km
Age: unknown

This prominent circular feature in the Sahara desert of Mauritania was initially interpreted as a meteorite impact structure because of its high degree of circularity and multiple ring configuration. Further investigation has shown that the structure probably formed by uplift and erosion of a sequence of sedimentary rocks, creating the series of annular rings, with the harder quartzites of the sequence forming ridges and the softer rocks eroding to form the intervening depressions. Despite thorough investigations, no traces of shock metamorphism or meteoritic signatures have been found at the structure. (Space shuttle photograph STS37-98-69.)

26. Impact Features

Location map for the terrestrial impact craters in this slide set. Numbers on the map correspond to numbers of the slide captions.

Glossary

Allocthonous - Material that is formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.

Asteroid - Any of the numerous small rocky bodies in orbit around the Sun. Most asteroids reside in the “main belt” between Mars and Jupiter, but some have orbits that cross the Earth’s orbit and could strike its surface.

Breccia - A coarse-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.

Cataclastic - A texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing.

Comet - One of the primitive icy bodies originating in the outer reaches of the solar system that are in elliptical orbits around the Sun. Near the Sun, the icy material vaporizes and streams off the comet, forming a glowing tail.

Cretaceous Period - A geological term denoting the interval of Earth history beginning around 144 million years ago and ending 66 million years ago.

Cretaceous-Tertiary boundary - A major stratigraphic boundary on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth.

Crystalline - Rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high-temperature or pressure environments, or igneous rocks that formed from cooling of a melt.

Diaplectic glass - A natural glass formed by shock pressure from any of several minerals without melting. It is found only in association with meteorite impact craters.

Ejecta - Material such as glass and fragmented rock thrown out of an impact crater during its formation.

High-pressure mineral phases - Mineral forms that are stable only at the extremely high pressures typical of Earth’s deep interior but not its surface. Such pressures are generated instantaneously during meteorite impact. For example, stishovite is the high-pressure polymorph of quartz, a common crustal mineral.

Hummocky - Uneven, "lumpy" terrain.

Impact melt - Rocks melted during impact, including small particles dispersed in various impact deposits and ejecta, and larger pools and sheets of melt that coalesce in low areas within the crater. Impact melts are extremely uniform in their composition but highly variable in texture. They are composed predominantly of the target rocks but may contain a small but measurable amount of the impactor.

Meteorite - An extraterrestrial rock that has fallen to Earth. Most meteorites are pieces of asteroids and are of stony, stony-iron, or iron composition.

Meteoroid - A small solid body moving through interplanetary space; after falling to Earth it is called a meteorite.

Paleozoic - A geological term denoting the time in Earth history between 570 and 245 million years ago.

Peak ring - A central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above about 50 km in diameter.

Planar features - Microscopic features in grains of quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain’s crystal structure.

Precambrian - A geological term denoting the time in Earth history prior to 570 million years ago.

Shield - Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth’s surface.

Shock metamorphism - The production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices.

Siderophile elements - Literally, “iron-loving” elements, such as iridium, osmium, platinum, and palladium, that, in chemically segregated asteroids and planets, are found in the metal-rich interiors. Consequently, these elements are extremely rare on Earth’s surface.

Stishovite - A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed quartzbearing rocks from meteorite impact craters.

Target rocks - The surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event.

Tektites - Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as droplets during terrestrial impact events. They range in color from black or dark brown to gray or green and most are spherical in shape. Tektites have been found in four regional deposits or “strewn fields” on the Earth’s surface: North America, Czechoslovakia (the moldavite tektites), Ivory Coast, and Australasia.